I can totally understand where you are coming from Sirio and definitely agree that cool down reduction should not be any larger than ~60%. I haven't got much experience with Eldevin, although I have played many other MMORPG's over the years. One such solution may be rather than adding a buff of 10% (ie Assassin buff) to the total percentage, it could be a percentage of the haste rating.
Bear in mind I am only level 13 so am not 100% sure how this translates to end game but...
If I have 248 haste rating = 9.3%
Then I use my Assassin buff -> 514 haste rating = 19.3%
What I am suggesting is perhaps the 10% buff should only increase the haste rating by 10% thus giving +24.8 haste rating.
This would be ~273 haste rating = ~10.2%
Again I am not sure how this would translate late game but it may be something that could be used even if that means increasing or decreasing the buff. The 10% buff would also increase haste by a lot more at level 40 when you have much higher haste rating. I would be interested to know whether the straight percentage increase would be more or less at level 40.